Adding new Exception for invocations of private methods

Currently Caller throws a MethodNotFound exception when a private methods is called
on the SUS, which prompts method generation logic.

This change introduces a new MethodNotVisible exception for cases when a private method is invoked.
Fixing incorrect spec for visibility exceptions
Merge pull request #224 from ciaranmcnulty/generating-private-methods

Adding new Exception for invocations of private methods